Call delivery in networks is accomplished in a variety of manners as a function of the environment. In one example, Technical Specifications (TS) 23.272 specifies steps that support LTE-to-1XRTT Circuit Switched Fall Back (CSFB) calling delivery and LTE-to-UMTS (Universal Mobile Telecommunication System)/GSM (Global System for Mobile Communications) CSFB. If the (UMTS/GSM) MSC (Mobile Switching Center) that receives a circuit voice page response differs from the MSC that sent the page request to the UE (User Equipment), the latter invokes “Roaming retry for CSFB”.
In LTE-to-UMTS/GSM CSFB, if the ISR (Idle mode Signaling Reduction) is enabled between the MSC and the underlay SGSN (Serving GPRS (General Packet Radio Service) Support Node), cross paging is performed if the UE is in an idle state, i.e., both the LTE and SGSN page the UE.